This invention relates generally to tractor-trailer vehicles and more particularly to a tractor suspension system which improves the ride quality of the tractor both when it is traveling empty and when it is pulling a fully loaded semi-trailer. The suspension system also extends the service life of the vehicle.
Over the road tractor-trailer vehicles have long been used to transport a wide variety of cargos. Typically, the tractor has a fifth wheel on which the front part of the semi-trailer rests. The fifth wheel is usually above the rear axle of the tractor. The load that is borne by the rear axle may be approximately 5,000 pounds when the tractor is traveling by itself. In contrast, when the tractor is pulling a fully loaded semi-trailer, the load on its rear axle may be on the order of about 25,000 pounds.
Due to this great variation in the loading on the rear axle of the tractor, considerable difficulty is presented in providing a suspension system for the tractor which gives reasonable ride quality when the tractor is traveling alone or with an empty semi-trailer while at the same time being able to handle a fully loaded semi-trailer. Presently available mechanical suspension components are unable to operate in a satisfactory manner throughout the wide range of loads that are encountered. Consequently, it is common practice at present to completely omit a suspension system for the tractor and, as can easily be appreciated, this results in a rough ride that can greatly discomfort the driver and other occupants of the vehicle as well as create considerable stresses on the tractor frame.
Attempts have been made to overcome this problem by providing a subframe which is pivoted to the main vehicle frame and which is connected with the axle. The load on the fifth wheel is applied directly to the subframe and through it to the axle. With this type of arrangement, suitable suspension elements can be provided between the axle and the tractor frame to cushion the forces that are transmitted from the axle to the frame. Examples of this type of structure are shown in U.S. Pat. Nos. 4,029,335 to Cady et al; 4,566,716 to Modat; and 3,315,980 to Gorge et al.
The subframes that have been proposed in the past are typically a pair of arms which are pivoted to the frame at one end and connected to the axle at the other end. The simple pivot connections that are provided for the arms allow the axle to move up and down to follow the contour of the roadway. However, if the axle is tilted transversely of the vehicle as occurs when the wheels on one end of the axle drop into a depression in the roadway or move over a raised area, the axle and subframe are subjected to twisting forces that can cause undue stressing of the components and create structural failure. A flexible type connection between the subframe arms and the axle is proposed in the above identified patents, but this complicates the construction of the suspension system and interposes additional springs or other suspension elements in the mounting structure for the fifth wheel.